Update dm worker intro

[alastori]

What is changed, added or deleted? (Required)

This PR updates the dm-worker-intro.md document to improve its clarity, accuracy, and completeness.

• Replaced "Features" with "Key Concepts": The previous "Features" list was inaccurate and confusing. It has been replaced with a "Key Concepts" section that correctly describes the worker's function, including its handling of full and incremental data, its high-availability capabilities, and the "one worker to one source" rule.
• Added a new Note block: A prominent note was added to clarify that DM-worker is a binlog client, not a standby replica, and to direct users to TiCDC for migrating data from a TiDB cluster.
• Expanded "Upstream Privileges" section: The original privilege list was insufficient for newer MariaDB versions. The documentation now provides separate, accurate privilege lists and GRANT examples for:
  • MySQL and MariaDB (< 10.5)
  • MariaDB (10.5 and later)
• Added workaround for MariaDB: A new note explains why the check-task command might fail with MariaDB 10.5+ and provides the ignore-checking-items workaround.

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see [tips for choosing the affected versions](https://github.com/pingcap/docs/blob/master/CONTRIBUTING.md#guideline-for-choosing-the-affected-versions).

• [x] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A
• Other reference link(s):
  • https://github.com/pingcap/tiflow/issues/12207
  • File changed: /dm/dm-worker-intro.md

Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

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[alastori]

cc @OliverS929